The invention involves the improvement of a continuous bauxite ore digestion process according to the Bayer process which enables the synchronised adjustment and optimisation of the temperature and the active caustic soda content in the aqueous medium to be used in digestion (also called attack liquor).
The invention also involves the improvement of the Bayer process digestion line used to recycle the weak caustic aqueous solution (called spent liquor) coming from the precipitation of aluminium trihydroxide, heating and concentrating it to an active caustic soda content complying with the objectives set by the User for bauxite digestion, in Bayer process digesters.
Throughout the description, the following definitions will apply:
the aqueous medium intended for use in digestion means both the spent liquor collected after the precipitation of Al(OH)3 which is recycled after heating it and concentrating the active caustic soda to form the attack liquor and the slurry formed by mixing this attack liquor with the crushed bauxite;
the aqueous medium from digestion means the liquor with a high aluminate content coming from bauxite digestion, along with possible unattacked sterile matter to be eliminated, also called pregnant liquor.
The Bayer process is the main technique for alumina production via the alkaline attack of bauxite. According to this process, which is continuous, the digestion of the bauxite is achieved by bringing about the solubilisation of the alumina under pressure and at an adequate temperature, in general at least 140xc2x0 C. (depending on whether digestion is carried out at a low or high temperature, the temperature level depends on the grade of bauxite processedxe2x80x94bayerite or hydrargillite or gibbsite, and diaspore or boehmite). This digestion is brought about by means of an aqueous caustic soda solution (called attack liquor) which has an appropriate concentration of active caustic soda (expressed hereafter in g/l of active Na2O). After digestion, an aqueous medium is obtained comprising a solution saturated in sodium aluminate (called pregnant liquor) and sterile impurities to be eliminated. This medium resulting from the digestion process is gradually cooled by multiple stage flash evaporation, then the pregnant liquor is separated from the phase of impurities which is discarded.
This phase of impurities makes up the unattacked residue of the ore (called red mud), which contains mainly various iron, silica and titanium oxides and metallic salts.
As for the pregnant liquor, it is cooled down to the required temperature and seeded with aluminium trihydroxide crystals to bring about the precipitation of the aluminium trihydroxide Al(OH)3 from the sodium aluminate. After liquid-solid separation, the separated aluminium trihydroxide undergoes calcination to produce alumina Al2O3 and the weak sodium aluminate liquor (called spent liquor) is sent back to be used in digestion after adequate heating and concentration of its active caustic soda content.
In the Bayer process as it is implemented today, substantial problems still exist involving both:
the amount of energy required to bring the attack liquor up to the temperature level set by the User;
the reconcentration of the active caustic soda content in the spent liquor (to the optimal level for digestion, and appropriate to the set temperature) by the evaporation of a quantity of water ensuring the balance of the process.
With regard to the quantity of heat required to raise the bauxite attack liquor to the set temperature level, the spent liquor is heated mainly by recovery of the sensible heat given off during the cooling of the pregnant liquor returning from digestion, in a multiple stage flash evaporation system. But this recovered energy provides the spent liquor with a certain temperature level which proves insufficient and which has to be further raised by using live steam in a series of tubular heaters.
To illustrate this phenomenon, in the case for instance of digestion at 141xc2x0 C., the spent liquor, which is heated and concentrated by evaporation in the heating zone (fed with the heat provided by the medium returning from digestion recovered by multiple stage flash evaporation) enters this zone at a temperature of between 80 and 90xc2x0 C. and leaves it at a temperature of about 120xc2x0 C.
This spent liquor, still not sufficiently heated, must therefore be raised to about 156xc2x0 C. (for example) so that the attack medium formed by the mixture of the said liquor and the bauxite (heated to about 90xc2x0 C.) reach the required temperature of 141xc2x0 C.: this rise in temperature from 120xc2x0 C. to 156xc2x0 C. is achieved by using live steam in tubular heat exchangers.
As for the reconcentration by evaporation of the water from the spent liquor, this is necessary due to the fact that this recycled liquor:
is diluted by intakes of water into the cycle, for example for washing operations on the impurities and the Al(OH)3 precipitate
must have an optimal active caustic soda content (expressed in g/l of active Na2O) for ore attack at the set temperature;
must be reconcentrated in such a way that the quantity of water present in the Bayer bauxite digestion cycle is kept as constant as possible.
This is why the excess water thus generated must be eliminated and this elimination is carried out by evaporation in at least one multiple stage flash evaporation zone, associated with a zone of tubular heat exchangers heated by flash vapour and, in addition, by live steam.
Thus, to heat and concentrate the spent liquor by evaporation, in the digester cycle of alumina production units (whether digestion is carried out at high or low temperatures) there is a multiple stage flash evaporation zone which enables the recovery of heat from the medium coming from digestion and its transfer to the medium intended for use in digestion (spent liquor) in a multiple stage flash evaporation zone made up of tubular heat exchangers in which the last stage is heated by live steam.
However, in addition to the flash evaporation zone previously mentioned, in many Bayer process alumina production units, there is also a (specific) evaporation section for the spent liquorxe2x80x94multiple stage flash (when the active caustic soda content is low) or multiple effect (in other cases), which completes the action of the said multiple stage flash evaporation zone. This specific evaporation section is fed with live steam and processes the spent liquor (which is at a temperature of about 80 to 90xc2x0 C.) resulting from the precipiation of Al(OH)3.